Method of preventing plugging of solution mining wells

ABSTRACT

Plugging of solution-mining wells by mineral precipitation due to solubility changes associated with temperature and pressure changes of the solution-mining liquid is prevented by injecting a diluent into the well production tubing, or into an area surrounding the production tubing intake.

[451 Feb. 19, 1974 METHOD OF PREVENTING PLUGGING OF 3,700,280 10/1972Papadopoulos......................... 299/5 SOLUTION MINING WELLS [75]Inventors: Lawrence H. Towell, Woodland Primary ExammerErnest R. PurserHills, Calif.; Jere R. Brew, Houston,

Tex.

Assignee: Shell Oil Company, Houston, Tex.

Filed: Aug. 14, 1972 ABSTRACT 21 Appl. No.: 280,287

mining wells by mineral precipitation due to solubility changesassociated with tempera- Plugging of solution- 52 us. Cl. 299/5.

ture and pressure changes of the solution-mining liquid is prevented byinjecting a diluent into the well production tubing, or into an areasurrounding the production tubing intake.

[56] References Cited I UNITED STATES PATENTS 3,523,582 8/1970 Fulford166/310 X 5 Claims, 7 Drawing Figures PAIENTE FEBI 91914 sum vu or 4MAX/MUM D/LUT/ON TEMPERATURE MAX/MUM PRODUCT/ON TEMPERATURE MIN/MUMPRODU C T/ON TEMPE RA TURE TEMPERATURE MIN/MUM D/LUTION I1 mmE mmmzmkFIG. 7

METHOD OF PREVENTING PLUGGING OF SOLUTION MINING WELLS BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to recoveringwater-soluble minerals from subsurface deposits by solution mining and,more particularly, to the recovery of water-soluble carbonates andbicarbonates from subsurface-oil shale formations by solution mining.

2. Description of the Prior Art Recovery of water soluble minerals suchas halite, trona, and nahcolite from subsurface deposits by solutionmining is well known. Commonly, water, or other mineral solvent, iscirculated through wells into mineral-containing formations to leachsoluble minerals therefrom. Since the solubility of most commerciallysolution-mined water-soluble minerals and the rate of dissolutionincreases with increasing temperature, it is usually desired to use aheated solvent in order to maximize the amount of mineral dissolved perunit of solution mining fluid flowed into the well.

However, as hot mineral-containing mining fluid flows upwardly in thewell, its temperature may drop due to heat losses in the wellbore.Additionally, its pressure drops due to a reduction in fluid head andthe effect of friction associated with flow. These changes can affectthe mineral carrying capacity of the mining fluid quite dramatically. Inmany cases, solubility is reduced to the extent that mineralprecipitation occurs. This may result in eventual plugging of theproduction well or of portions of the mineral containing formation ofrelatively lower temperature than the temperature of injectedsolution-mining fluid.

SUMMARY OF THE INVENTION We have now discovered an improved method forproducing water soluble minerals by solution mining in whichprecipitation of minerals in and around the production well due totemperature and pressure changes is prevented. The improvementcomprises-injecting a diluent directly into the flow stream ofmineral-laden solution-mining fluid at a point adjacent the bottom ofthe well under controlled conditions to prevent precipitation ofminerals within the flow area of diluted solution-mining fluid. Thediluent, advantageously, is an aqueous fluid and, preferably, is hotwater of low alkalinity. The diluent may be injected continuously orintermittently directly into a production tubing string or into an areain communication with the production tubing string inlet.

In applying the invention in the solution mining of heat-sensitivebicarbonate minerals, such as trona or nahcolite, particular care mustbe taken to maintain the diluent temperature within specific controlledranges to prevent deposition of carbonate minerals at one temperatureextreme or bicarbonate minerals at the other temperature extreme.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-5 are sectional viewsillustrating various embodiments of well equipment suitable for thepractice of this invention.

FIG. 6 is a graphical representation of sodium carbonate and sodiumbicarbonate concentration expressed as equivalent sodium bicarbonateconcentration, as a function of temperature and pressure in a sodiumcarbonate, sodium bicarbonate, water system.

FIG. 7 is a graphical representation illustrating maximum and minimumdilution fluid temperature and maximum and minimum total dilutedsolution-mining fluid temperature as a function of dilution ratio for aparticular nahcolite-water system at a pressure of 600 psi andoriginally at a temperature of 4l0F.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIGS. l5, in whichlike numerals represent like elements, we see a subsurface earthformation 10 containing at least one zone or layer 11 rich in at leastone water-soluble mineral such as nahcolite. Nahcolite zone 11 ispenetrated by at least one well 12 which has been completed in aconventional manner with cemented casing 13. The casing is provided withone or more preforations 14 for opening the interior of the casing intocommunication with the nahcolite zone 11. The well is provided with aproduction tubing string 15 for flowing mineral-containing fluid fromthe well. Additionally, a fluid flow path is provided for injectingdilution fluid from the surface into the well to a point adjacent thebottom of the production tubing 15. In the embodiments of FIGS. 1, 2 and3, this flow path is formed by a separate dilution fluid conduit l6, l7and 18, respectively. In the embodiments of FIGS. 4 and 5, this flowpath is provided by the annular space 19 between the casing 13 and theproduction tubing 15.

This annular space 19 is closed by pack-off means 23 above theperforations 14 through which fluid is produced from formation 10.Suitable flow regulating means, such as pressure actuated valve 21 (FIG.4) or flow control orifice 22 (FIG. 5), is provided for regulating theflow of dilution fluid into the production tubing string at a pointadjacent the lower end thereof. Flow through such flow regulating meanscan be controlled by adjusting the pressure in the annular spacesurrounding the production tubing 15. For example, a pressurized gas maybe maintained in this annulus, the pressure being adjusted to regulatethe rate of flow through the pressure actuated value 21, which may be aspring loaded check valve, or through orifice 22.

To remove nahcolite from the zone 11, a hot solution mining fluid, suchas hot water or steam, is flowed into contact with the zone 11 to leachnahcolite therefrom. This hot, now mineral-laden, fluid then flows intothe casing 13 of the well through preforations l4 and is lifted to thesurface through production tubing 15.

The solution-mining fluid may be injected through the well 12 inalternating sequence with the production of fluids according to themethod of this invention, or it may be injected simultaneously withfluid production. For example, in well equipped as shown in FIGS. 1-3,the solution mining fluid may be injected through the space 19surrounding the dilution fluid injection string 16-18 and the productiontubing string 15. This fluid may flow into contact with the nahcolitezone 11 through a set of perforations 20 in the casing 13 near the topof formation 10. In another embodiment, hot solution mining fluid may beinjected through a separate injection well (not shown), flowed throughthe formation l0, and the produced up the well 12. FIGS. 4 and 5illustrate equipment configurations well suited to this productionscheme.

As mineral-containing, solution-mining fluid rises to the surfacethrough production tubing 15, its temperature falls due to heat lossesto material in the well 12 and to surrounding formations.Simultaneously, its pressure falls due to reduction of fluid head. Thesetemperature and pressure changes can reduce the capacity of the solutionmining fluid to carry dissolved minerals. This may result in mineralprecipitation in the tubing 15.

For example, FIG. 6 shows dissolved sodium bicarbonate and sodiumcarbonate concentration expressed in' equivalent pounds of Nal-ICO perpound of water as a function of hot water temperature and pressure for anahcolite-water system (i.e. any Na- CO content is expressed in terms ofthe number of pounds of Nal-ICO containing the same amount of Na). Ifsolution-mining water contacting zone 11 is at a temperature of 325F andpressure of 1,500 psi, the water can carry 0.42 pounds/pounds ofdissolved sodium bicarbonate (NaH- CO If at the earth surface pressurehas been reduced to 500 psi and temperature to 290F, the liquid can thencarry only 0.35 pounds/pound. It follows that 0.07 pounds per pound ofsodium mineral will be precipitated within the production tubing as thesolution mining fluid rises to the surface if no steps are taken toprevent this precipitation. Therefore, according to the method of thisinvention, a diluent such as fresh water is introduced into the flowstream of substantially saturated solution-mining fluid downhole nearthe production tubing intake 24. For example, the diluent can beco-mingled with the solution-mining fluid at the points shown in any ofFIGS. l-S to dilute the produced fluid before it is cooled or subjectedto pressure reductions which would cause the precipitation of dissolvedminerals.

Preferably, the fresh water diluent has a very low level of hardness(e.g., contains substantially no calcium or magnesium ions) in order toprevent downhole carbonate scale deposition. In solution mining ofsodium minerals, such as nahcolite, the dilution water should have arelatively low level of alkalinity as well, so that fluid handlingproblems are not compounded by the fact that large volumes of water arenecessary to reduce alkalinity.

The diluent is preferably added in sufficient amount to substantiallyprevent mineral precipitation in production tubing 15. The amount ofdiluent required will vary with changes in diluent temperature. Wherethe solubility of the mineral being mined increases with increasedtemperature, less diluent will be required as diluent temperature isincreased.

In many cases it is desirable to heat dilution fluid prior to injectiondown the dilution tubing string 14. Heated dilution fluid will reducethe temperature reduction in the production tubing string 15 and therebyreduce the tendency of precipitate to form within this string. This, inturn, reduces the amount of diluent required to prevent precipitation.

Advantageously, heated dilution fluid is flowed into the well 12 incounter-current heat exchange relationship with produced solution-miningfluids. The dilution fluid heats the produced fluid in tubing 15 byconduction as it passes down the dilution fluid tubing 14. This heatingfurther supplements precipitation preventing effects from diluentinjection.

In this regard the embodiment of FIG. 1 wherein dilution water is floweddown a tubing string 16 concentric with the production tubing string 15is particularly preferred. This configuration provides an ideal tube-inshell heat exchanger. For most practical operating parameters, thedilution string temperature becomes the same as the temperature of thefluid entering the production tubing intake. In such a situationsufficient diluent should be added to dilute the produced fluid streamadjacent the bottom of production tubing 15 to a mineral content lowerthan saturation content at the earth surface. For instance, in the abovementioned example, if the temperature of the diluent added is such thatno temperature changes in the system are created by addition of thisdiluent, then no deposition will occur in the production tubing 15 aslong as sufficient diluent is added to reduce the combined NaHCO and NaCO content of the produced fluid in the production tubing 15 adjacentthe bottom of the tubing 15 to 0.35 or less pounds of sodium bicarbonateper pound of water.

Where the dilution fluid added is at a temperature lower than that ofthe solution-mining fluid with which it is inter-mixed, care must betaken to ensure that the temperature decreasing effect of the dilutionfluid does not diminish the capacity of the total mixture to carrywater-soluble minerals more than the increased carrying capacityattributable to the presence of dilution fluid. FIG. 7 illustrates thisfor a nahcolite system at 600 psi with an initial solution mining fluidsodium concentration of 1.3 pounds/pound and an initial temperature of410F. The lower-most curve gives minimum temperature of the dilutionfluid as a function of dilution ratio. The adjacent curve gives minimumtemperature of the mixed fluid stream which may be attained withoutprecipitation. It can be seen that a 2:1 dilution ratio precipitationwill occur if the dilution fluid temperature is below 220F or if thetemperature of the mixed stream falls below 280F. It can also be seenthat at larger dilution ratios precipitation can be avoided by adding adiluent of substantially lower temperature.

In dealing with heat-sensitive water-soluble minerals, care must betaken not only to ensure that minimum temperatures are maintained for agiven dilution ratio but also to stay below certain maximum temperaturesat that ratio. FIG. 6 reflects the fact that in a nahcolite (Nal-ICOwater system with increases in temperature or decreases in pressure,nahcolite decomposes to sodium carbonate (Na cfi and carbon dioxide (COFor example, considering a system having a combined sodium bicarbonate,sodium carbonate concentration of 0.8 pounds/pound and a pressure of 500psi, FIG. 6 indicates that at 340F the solution is saturated at 0.72pounds/pound. Therefore, sodium bicarbonate will precipitate. Iftemperature is raised to 350F the system becomes capable of dissolvingas much as 0.85 pounds/pound; therefore, there will be no precipitation.However, if temperature is increased to 410F, Na CO concentration due todecomposition of NaHCO has increased to the extent that the system isonce again saturated. At temperatures above 4l0F the system issuper-saturated with NaCO and precipitation of this mineral will occur.

Thus, as illustrated by the upper two curves of FIG. 7 for that sodiumbicarbonate system, the dilution fluid temperature must be maintainedbelow a certain maximum given by the uppermost curve to prevent Na COdeposition upon the addition of diluent to a solution of nahcolite inwater. The adjacent lower curve gives the maximum temperature of themixed fluid stream as a function of dilution ratio. For the 2:1 examplediscussed above, the dilution fluid temperature must be above about 220Fbut below about 530F and the temperature of the diluted production fluidmust be in the range of about 300480F.

The particular temperature range for a given heatsensitive mineralmining process varies with changes in system pressure and dilutionratio. Given solubility in formation such as that of FIG. 6, one skilledin the art should have no problem determining particular temperaturelimits for a given heat-sensitive mineral-water system. It should bepointed out that in high tempera ture nahcolite systems, solubility isquite sensitive to pressure, thus, at 450F a given volume of water candissolve 0.47 pounds of sodium bicarbonate per pound of water at 500 psiand 1.22 pounds of sodium bicarbonate per pound of water at 1,800 psi.Thus, one must be careful in designing a system in which fairly largepressure decreases are incurred as high temperature fluids are lifted ina well bore to ensure that from point to point along the path ofconditions through which the solution passes, no precipitation occurs.

The method of solution-mining water soluble minerals heretoforedescribed is particularly applicable to insitu oil-shale processingprojects in which nahcolite or other water soluble mineral is leachedfrom a nahcolite-rich oil shale formation in conjunction with ahydrocarbon recovery process. There, leaching fluid temperature may bequite high (250550F or higher) because the solution-mining fluid, inaddition to leaching nahcolite, may be simultaneously used as anoilshale thermal-fracturing agent or as a kerogenpyrolyzing fluid. Heatlosses can be quite severe as this high temperature fluid is withdrawnfrom a subsurface formation through a well. Dilution of this fluid withfresh water prevents well equipment plugging by mineral precipitation.

We claim as our invention:

1. In a method of mining from a subsurface earth formation a heatsensitive bicarbonate which decomposes to solution with increasingtemperature, wherein heated water is flowed into contact with saidbicarbonate to remove said bicarbonate from the formation by dissolutionand wherein the resulting bicarbonatecontaining aqueous solution islifted to the earth surface through a well, the improvement whichcomprises, prior to lifting said aqueous solution to the earth surface,

mixing water with said aqueous solution at a point adjacent the bottomof the well at temperature high enough to prevent bicarbonate depositionin the well but lower than a maximum temperature at which carbonateprecipitation occurs. 2. The method of claim 1 wherein the well is casedwith a tubular casing and said bicarbonatecontaining solutio is liftedthrough a production tubing string within the casingv and including thesteps of:

positioning pack-off means in the space between the casing and theproduction tubing at a point above the inlet of saidbicarbonate-containing solution into the production tubing;

providing pressure-responsive flow control means in the productiontubing string above the pack-off means for admitting water into theproduction tubing from the space surrounding the production tubing;

injecting water into the well through the space between the productiontubing and the casing;

and adjusting the rate at which water flows through thepressure-responsive flow control means by adjusting pressure in thespace between the production tubing and casing.

3. The method of claim 1 which includes, prior to mixing, the step ofheating the water to a temperature at least as high as the temperatureof said bicarbonatecontaining aqueous solution with which the water isto be mixed.

4. In a method for recovering nahcolite and hydrocarbons from asubsurface, nahcolite-containing oilshale formation of the type whereinthe oil-shale is permeabilized by injecting an aqueous nahcolite solventinto contact with the formation at a temperature sufficient to thermallyfracture the oil shale and wherein hot nahcolite-containing nahcolitesolvent is withdrawn from the formation through a well production tubingstring, the improvement comprising:

injecting water into the lower portion of the production tubing stringat a temperature high enough to prevent bicarbonate deposition in thewell but lower than a maximum temperature at which carbonateprecipitation occurs.

5. The method of claim 4 wherein the water contains substantially nocalcium or magnesium ions.

2. The method of claim 1 wherein the well is cased with a tubular casingand said bicarbonate-containing solution is lifted through a productiontubing string within the casing and including the steps of: positioningpack-off means in the space between the casing and the production tubingat a point above the inlet of said bicarbonate-containing solution intothe production tubing; providing pressure-responsive flow control meansin the production tubing string above the pack-off means for admittingwater into the production tubing from the space surrounding theproduction tubing; injecting water into the well through the spacebetween the production tubing and the casing; and adjusting the rate atwhich water flows through the pressure-responsive flow control means byadjusting pressure in the space between the production tubing andcasing.
 3. The method of claim 1 which includes, prior to mixing, thestep of heating the water to a temperature at least as high as thetemperature of said bicarbonate-containing aqueous solution with whichthe water is to be mixed.
 4. In a method for recovering nahcolite andhydrocarbons from a subsurface, nahcolite-containing oil-shale formationof the type wherein the oil-shale is permeabilized by injecting anaqueous nahcolite solvent into contact with the formation at atemperature sufficient to thermally fracture the oil shale and whereinhot nahcolite-containing nahcolite solvent is withdrawn from theformation through a well production tubing string, the improvementcomprising: injecting water into the lower portion of the productiontubing string at a temperature high enough to prevent bicarbonatedeposition in the well but lower than a maximum temperature at whichcarbonate precipitation occurs.
 5. The method of claim 4 wherein thewater contains substantially no calcium or magnesium ions.